linux-kernel - Re: fw_devlinks preventing a panel driver from probing

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Message-ID: <34mewzvfyjsvyud3lzy6swxs7sr64xkgrbkxtvyw3czvoerct7@7guo32ehwa52>
Date: Sat, 21 Sep 2024 23:15:10 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	"Rafael J. Wysocki" <rafael@...nel.org>, Linux Kernel List <linux-kernel@...r.kernel.org>, 
	Saravana Kannan <saravanak@...gle.com>, Aradhya Bhatia <aradhya.bhatia@...ux.dev>, 
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>, Devarsh Thakkar <devarsht@...com>
Subject: Re: fw_devlinks preventing a panel driver from probing

On Mon, Sep 16, 2024 at 02:51:57PM GMT, Tomi Valkeinen wrote:
> Hi,
> 
> We have an issue where two devices have dependencies to each other,
> according to drivers/base/core.c's fw_devlinks, and this prevents them from
> probing. I've been adding debugging to the core.c, but so far I don't quite
> grasp the issue, so I thought to ask. Maybe someone can instantly say that
> this just won't work...

Well, just 2c from my side. I consider that fw_devlink adds devlinks for
of-graph nodes to be a bug. It doesn't know about the actual direction
of dependencies between corresponding devices or about the actual
relationship between drivers. It results in a loop which is then broken
in some way. Sometimes this works. Sometimes it doesn't. Sometimes this
hides actual dependencies between devices. I tried reverting offending
parts of devlink, but this attempt failed.

> 
> If I understand the fw_devlink code correctly, in a normal case the links
> formed with media graphs are marked as a cycle (FWLINK_FLAG_CYCLE), and then
> ignored as far as probing goes.
> 
> What we see here is that when using a single-link OLDI panel, the panel
> driver's probe never gets called, as it depends on the OLDI, and the link
> between the panel and the OLDI is not a cycle.

I think in your case you should be able to fix the issue by using the
FWNODE_FLAG_NOT_DEVICE, which is intented to be used in such cases. You
have a dependency on DT node which doesn't have backing device.

> 
> The DSS driver probes, but the probe fails as it requires all the panel
> devices to have been probed (and thus registered to the DRM framework)
> before it can finish its setup.
> 
-- 
With best wishes
Dmitry